I. Field of the Invention
The present invention relates to a communications system. More particularly, the present invention relates to an apparatus and method for adding and removing a cell-site base station from a cellular communications system when system loads increase or decrease or when base station maintenance is required.
II. Description of the Related Art
In some cellular telephone systems, personal communications systems and wireless local loop systems using code division multiple access (CDMA) coding technique, a common frequency band is used for communication with all base stations in the system. The common frequency band allows simultaneous communication between a mobile unit and more than one base station. Signals occupying the common frequency band are discriminated at the receiving terminal (either within the base station or mobile unit) through the spread spectrum CDMA waveform properties based on the use of high frequency pseudonoise (PN) codes and orthogonal Walsh codes. Transmitting terminals (either the base station or mobile unit) using different PN codes or PN codes that are offset in time or orthogonal Walsh codes produce signals that can be separately received at the receiving terminal.
In an exemplary CDMA system, each base station transmits a pilot signal having a common PN spreading code that is offset in code phase from the pilot signal of other base stations within the system. During system operation, the mobile unit is provided with a list of code phase offsets corresponding to neighboring base stations surrounding the base station through which communication is established. The mobile unit is equipped with a searching element that allows the mobile unit to track the signal strength of the pilot signal from a group of base stations including the neighboring base stations.
Various methods exist for switching the mobile unit from one base station to another (known as "handoff"). One such method is termed a "soft" handoff, in which communication between the mobile unit and the end user is uninterrupted by the eventual handoff from an original base station to a subsequent base station. This method is considered a soft handoff in that communication with the subsequent base station is established before terminating communication with the original base station. When the mobile unit is communicating with two base stations, a single signal for the end user is created from the signals from each base station by a cellular or personal communication system controller. U.S. Pat. No. 5,267,261 which is incorporated by this reference and assigned to the assignee of the present invention, discloses a method and system for providing communication with the mobile unit through more than one base station during the handoff process, i.e., providing soft handoff.
Mobile unit assisted soft handoff operates based on the pilot signal strength of several sets of base stations as measured by the mobile unit. An Active Set is the set of base stations through which active communication is established. A Neighbor Set is a set of base stations surrounding an active base station comprising base stations that have a high probability of having a pilot signal strength of sufficient level to establish communication. A Candidate Set is a set of base stations having a pilot signal strength of sufficient level to establish communication.
When communications are initially established, a mobile unit communicates through a first base station, and the Active Set contains only the first base station. The mobile unit monitors the pilot signal strength of the base stations in the Active Set, the Candidate Set, and the Neighbor Set. When a pilot signal of a base station in the Neighbor Set exceeds a predetermined threshold level, the base station is added to the Candidate Set and removed from the Neighbor Set at the mobile unit. The mobile unit communicates a message identifying the new base station. A cellular or personal communication system controller decides whether to establish communication between the new base station and the mobile unit. Should the cellular or personal communication system controller decide to do so, the cellular or personal communication system controller sends a message to the new base station with identifying information about the mobile unit and a command to establish communications with the mobile unit. A message is also transmitted to the mobile unit through the first base station. The message identifies a new Active Set that includes the first and new base stations. The mobile unit searches for the new base station's transmitted information signal, and communication is established with the new base station without termination of communication through the first base station. This process can continue with additional base stations.
When the mobile unit is communicating through multiple base stations, it continues to monitor the signal strength of the base stations of the Active Set, the Candidate Set, and the Neighbor Set. Should the signal strength corresponding to a base station of the Active Set drop below a predetermined threshold for a predetermined period of time, the mobile unit generates and transmits a message to report the event. The cellular or personal communication system controller receives this message through at least one of the base stations with which the mobile unit is communicating. The cellular or personal communication system controller may decide to terminate communications through the base station having a weak pilot signal strength.
Upon deciding to terminate communications through a base station, the cellular or personal communication system controller generates a message identifying a new Active Set of base stations. The new Active Set does not contain the base station through which communication is to be terminated. The base stations through which communication is established send a message to the mobile unit. The mobile unit communications are thus routed only through base stations identified in the new Active Set.
Because the mobile unit is communicating with the end user through at least one base station at all times throughout the soft handoff processes, no interruption in communications occurs between the mobile unit and the end user. A soft handoff provides significant benefits in its inherent "make before break" communication over conventional "break before make" techniques employed in other cellular communication systems.
In a cellular or personal communication telephone system, maximizing the capacity of the system in terms of the number of simultaneous telephone calls that can be handled is extremely important. System capacity in a spread spectrum system can be maximized if the transmitter power of each mobile unit is controlled such that each transmitted signal arrives at the base station receiver at the minimum level required to maintain the link. If a signal transmitted by a mobile unit arrives at the base station receiver at a power level that is too low, the bit-error-rate may be too high to permit high quality communications due to interference from the other mobile units. If, on the other hand, the mobile unit transmitted signal is at a power level that is too high when received at the base station, communication with this particular mobile unit is acceptable, but this high power signal acts as interference to other mobile units. This interference may adversely affect communications with other mobile units.
Path loss in the radio channel is defined as any degradation or loss suffered by a signal as it travels over-the-air and can be characterized by two separate phenomena: average path loss and fading. The forward link, i.e., the link from the base station to the mobile unit, typically but not necessarily operates on a different frequency than the reverse link, i.e., the link from the mobile trait to the base station. Nevertheless, because the forward and reverse link frequencies are within the same frequency band, a significant correlation exists between the average path loss of the two links. For example, a typically cellular system has one of its forward link channels centered about 882 MHz paired with one of its reverse link channels centered about 837 MHz. On the other hand, fading is an independent phenomenon for the forward link and reverse link and varies as a function of time. The characteristics of fading on the channel are the same, however, for both the forward and reverse link because the frequencies are within the same frequency band. Therefore, the average of channel fading over time for both links is typically the same.
In an exemplary CDMA system, each mobile unit estimates the path loss of the forward link based on the total power at the input to the mobile unit. The total power is sum of the power from all base stations operating on the same frequency assignment as perceived by the mobile unit. From the estimate of the average forward link path loss, the mobile unit sets the transmit level of the reverse link signal.
Mobile unit transmit power is also controlled by one or more base stations. Each base station with which the mobile unit is communicating measures the received signal strength from the mobile unit. The measured signal strength is compared to a desired signal strength level for that particular mobile unit at that base station. A power adjustment command is generated by each base station and sent to the mobile unit on the forward link. In response to the base station power adjustment commands, the mobile unit increases or decreases the mobile unit transmit power by a predetermined amount.
When a mobile unit is in communication with more than one base station, power adjustment commands are provided from each base station. The mobile unit acts upon these multiple base station power adjustment commands to avoid transmit power levels that may adversely interfere with other mobile unit communications and yet provide sufficient power to support communication from the mobile unit to at least one of the base stations. This power control mechanism is accomplished by having the mobile unit increase its transmit signal level only if every base station with which the mobile unit is communicating requests an increase in power level. The mobile unit decreases its transmit signal level if any base station with which the mobile unit is communicating requests that the power be decreased. A system for base station and mobile unit power control is disclosed in U.S. Pat. Nos. 5,056,109, 5,265,119, 5,257,283, and 5,267,262; all of which are incorporated by this reference and assigned to the assignee of the present invention.
Base station diversity at the mobile unit is an important consideration in the soft handoff process. The power control method described above operates optimally when the mobile unit communicates with each base station through which communication is possible, typically between one to three base stations although a greater number is possible. In so doing, the mobile unit avoids inadvertently interfering with communications through a base station that is receiving the mobile unit's signal at an excessive level, but that cannot communicate a power adjustment command to the mobile unit because communication is not established with the mobile unit.
Each base station coverage area has two handoff boundaries. A handoff boundary is defined as the physical location between two base stations where the link would perform the same regardless of whether the mobile unit were communicating with the first or second base station. Each base station has a forward link handoff boundary and a reverse link handoff boundary. The forward link handoff boundary is defined as the location where the mobile unit's receiver would perform the same regardless of which base station it was receiving. The reverse link handoff boundary is defined as the location of the mobile unit where two base station receivers would perform the same with respect to that mobile unit.
Ideally these boundaries should be balanced, meaning that they have the same physical location with respect to the base station. If they are not balanced, system capacity may be reduced as the power control process is disturbed or the handoff region unreasonably expands. Note that handoff boundary balance is a function of time, in that the reverse link power increases as the number of mobile units increases. Reverse link power is inversely proportional to coverage area. Therefore all other conditions remaining static, an increase in reverse link power decreases the effective size of the coverage area of the base station and causes the reverse link handoff boundary to move inward towards the base station. Unless a compensation mechanism for the forward link is incorporated in the base station, even a system that is initially perfectly balanced will be unbalanced intermittently depending on the loading of the base station.
In a working cellular, personal communication, or wireless local loop system, fluctuation in loading are common. For example, if an accident occurs on a major freeway at rush hour, the resultant traffic jam can cause a substantial increase in the number of system users trying to access the system. Planned events, such as large sporting events, conferences, and parades, can have the same effect. A large fluctuation in loading that increases the number of users far beyond the average expected loading may overload the system. If the overload is substantial, requests for new communication links must be denied. Although the overload situation is undesirable, the obvious alternative of providing additional capacity to every base station in the system is impractical. Presently, however, no such method or apparatus exists by which overload situations can be avoided without temporarily interrupting or degrading system performance.
Furthermore, when a base station requires routine or unexpected maintenance, the base station must be removed from the system and replaced when maintenance is complete. It is important in removing and replacing the base station, however, to maintain normal operation of the system and to prevent interruption of any ongoing system communications. Conventional systems, however, provide no means by which a base station can be removed from and returned to the system when the base station requires maintenance without deleterious effects on system performance.
Therefore, a need exists for an apparatus and method for efficiently handling and avoiding overload situations and for maintaining normal system operations when performing base station maintenance.